1. Field of the Invention
The present invention relates to compositions and methods for inhibiting deposits during calcination of fluxed iron ore pellets in which the fluxing materials have a phosphate content of less than 1% by weight of the total weight of fluxing material and iron ore.
Crude iron ore cannot be used directly in the steel making process, but must first be concentrated and refined. When the iron content of the ore is increased, the process generally is referred to as concentration, and this can sometimes be accomplished simply by crushing, screening, and washing. Other times, the ore is ground to very small particles before the iron oxides can be separated from the rest of the material, called gangue, which is normally accomplished by magnetic drums.
However, even where there is satisfactory concentration, iron ore consisting of fine particles is undesirable as blast-furnace feed; the iron ore particles must first be agglomerated into a coarser form, and this process is referred to as agglomeration. The most desirable size for blast-furnace feed is from 6-25 mm, and pelleting is one of the methods frequently used to achieve this type of coarse iron ore feed.
In the pelletizing process, which accounts for about two-thirds of U.S. agglomerate production, the ore must be ground to a very fine size, less than 75 .mu.m. The ground ore is mixed with the proper amount of water, and sometimes with a small amount of bentonite, and this is rolled into small balls 10-20 mm in diameter in a balling drum or disk. These green pellets are dried, then are heated to 1200.degree.-1370.degree. C. to bond the small particles, and finally are cooled. The heating can be done on a traveling grate, or in a shaft furnace, or by a combination of a traveling grate and a rotary kiln.
Another of the chief raw materials in the steel making process in addition to the iron ore, is the fluxing material consisting of lime (CaCO.sub.3) and/or dolomite (CaCO.sub.3 -MgCO.sub.3). Typically, limestone is crushed and screened to the desired particle size, and burnt lime for steel making is then prepared from the limestone by calcination in a long rotary kiln. It is common to combine the iron ore pelletizing operation described above with the limestone and/or dolomite flux preparation and calcination by adding the limestone and/or dolomite particles directly to the iron ore particles which are to be formed into pellets. This mixture is then heated in the same device, usually a long rotary kiln, often with a traveling grate, so that the pelletizing and limestone and/or dolomite calcination are accomplished in the same step and in the same heating furnace. This combined step is usually referred to as calcination of the iron ore, although the chief result is the hardening of the green iron ore pellets.
During the heating of the mixture of particles of limestone and/or dolomite flux and particles of iron ore formed into pellets, which will be referred to as flux pellet kilning, a problem is frequently encountered involving deposits which form on the walls of the rotary kiln or other furnace or heating device being used. These deposits are formed as a result of the flux pellet kilning operation, perhaps as the result of a combination of mechanical adhesion and condensation on the cooler skin of the kiln or furnace surface. The predominant constituent of such deposits is ferric oxide (hematite), with the majority of the remainder being magnetic iron oxide (magnetite). However, there is frequently a significant amount, about 2-10% by weight of the total deposit, of calcium phosphate, Ca.sub.10 (PO.sub.4).sub.6 (OH).sub.2 (hydroxyapetite).
Such deposits create substantial problems in the kilning operation, e.g., by forming a barrier which resists the transfer of heat, thus reducing the efficiency and thereby increasing the heating time required. Also, large portions of such deposits can break away and become admixed with the pellets being calcined, thus resulting in an unacceptable final product. As a result of the formation of these deposits, significant removal problems are created.
For example, there is a significant down time for the kilns, furnaces or other heating devices being used, during which the deposits are mechanically removed by such off-line cleaning methods as compressed air driven jack-hammers, small charges of blasting explosives, or more time-consuming approaches utilizing hammers and chisels, etc. These processes of mechanical removal present serious problems in addition to the down time which they entail. An on-line method of cleaning which is frequency used involves mechanical removal of these deposits by "shooting", in which the deposits are blasted away by repeated discharging of shotguns against the deposits. This procedure poses the obvious risks to the personnel performing it, but also has been known to result in serious damage to the walls of the kiln or other furnace heating device being used.
In order to significantly inhibit the formation of these flux pellet kiln deposits, and thereby significantly increase the efficiency of the flux pellet kilning operation, the present invention provides for the administration in liquid form of a stable aqueous suspension of a combination of magnesium hydroxide and copper oxychloride, together with a suspending agent, described in detail further below, to the flux pellet kilning operation.
However, it has been found that the method of the present invention will not satisfactorily inhibit deposits in the flux pellet kilning operation where the flux being used has too high a phosphate content. Analysis of deposits, particularly with respect to the presence of high amounts of calcium phosphate (hydroxyapetite), and correlation thereof with unsatisfactory inhibition, has led to the discovery that a critical limitation for preventing such deposits is that the phosphate content, as P.sub.2 O.sub.5, must be less than 1% by weight of the total weight of the flux pellet, i.e., the combined weight of iron ore and flux material.
2. Brief Description of the Prior Art
The use of copper oxychloride to inhibit deposit formation is disclosed in Kiss, L. T., et al., "The Use of Copperoxychloride to Alleviate Boiler Slagging", Journal of the Institute of Fuel, April 1972, pages 213-223.
U.S. Pat. No. 2,845,338 discloses the use of blends of magnesium oxide and copper oxychloride to inhibit deposits in coal-fired boiler furnaces. The principal deposits are CaO (generally less than 10%), SiO.sub.2, Al.sub.2 O.sub.3, and Fe.sub.2 O.sub.3.
U.S. Pat. No. 3,404,099 describes making of catalyst compositions by precipitating iron oxychloride and then calcining it.
U.S. Pat. No. 4,372,782 describes production of high grade lead and silver involving treatment with brine and lime to produce oxychlorides followed by calcination.
U.S. Pat. No. 4,503,019 discloses the use of blends of magnesium oxide and copper oxychloride for inhibiting and dispersing calcium oxide deposit formation in coal-fired kilns.
U.S. Pat. No. 4,561,897 describes an aqueous magnesium hydroxide suspension dispersible in oil, and used to inhibit corrosion in fuel burning boilers.
Blends of magnesium hydroxide and copper oxychloride have been used to prevent deposit formation in ordinary unfluxed iron ore pelletizing operations, but not successfully to flux pellet kilning.
None of the above applications in any way suggest the compositions and methods of the present invention.